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Resonance Raman spectroscopy is a Raman spectroscopy technique in which the incident photon energy is close in energy to an electronic transition of a compound or material under examination. The frequency coincidence can lead to greatly enhanced intensity of the Raman scattering, which facilitates the study of chemical compounds present at low concentrations.
Raman scattering is usually extremely weak, since only about 1 in 10 million photons that hit a sample are scattered with a loss or gain in energy from changes in vibrational energy of the molecules in the sample; the rest of the photons are scattered with no change in energy. Resonance enhancement of Raman scattering requires the incident wavelength to be close to that of an electronic transition of the molecules. In larger molecules the change in electron density can be largely confined to one part of the molecule, a chromophore, and in these cases the Raman bands that are enhanced are primarily from those parts of the molecule in which the electronic transition leads to a change in bond length or force constant in the excited state of the chromophore. For large molecules such as proteins, this selectivity helps to identify the observed bands as originating from vibrational modes of specific parts of the molecule or protein, such as the heme unit within myoglobin.